Heat sink



FIG. 1 is a front elevational view of the new design for a heat sink, the design being rotationally symmetric;

FIG. 2 is a top plan view thereof;

FIG. 3 is a bottom plan view thereof; and,

FIG. 4 is a perspective view thereof as seen from the front, top side.

The broken line disclosure is for environmental purposes only and does not form a part of the claimed design. 

The ornamental design for a heat sink, as shown and described. 